Communication devices such as wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a wireless communications system or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications system.
Wireless devices may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as wireless device or a server.
The wireless communications system covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the area of radio coverage provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP GSM EDGE Radio Access Network (GERAN), a wireless device has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. EDGE is an abbreviation for Enhanced Data rates for GSM Evolution. In the end of 4008 the first release, Release 8, of the 3GPP Long Term Evolution (LTE) standard was finalized and later releases have also been finalized.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In cellular telecommunications, the term handover refers to the process of transferring an ongoing call or data session from one cell serviced by a network node connected to the core network, i.e., from a source cell serviced by a source network node, to another cell serviced a another network node, i.e., to a target cell serviced by a target network node. In a typical wireless communications network, one network node only covers one or more limited geographical area or cell; therefore, handover from a source cell to a target cell becomes a very important feature for the seamless mobility of wireless devices in the entire wireless communications network. The performance of handover also becomes an important factor that affects the user's experience.
As shown schematically in FIG. 1, in current systems, the general handover process comprises three sub-processes:
(1) When a wireless device detects a better cell which fulfills a certain requirement for handover, the wireless device sends a measurement report which comprises target cell information to inform the source network node to trigger handover preparation. Handover requirements include an offset between source cell and target cell and time delay to trigger the measurement report.
(2) When the source network node receives the measurement report, it will coordinate with the target network node which serves the target or neighboring cell, if there is available resource for the wireless device. The source network node will then send an RrcReconfiguration message to the wireless device, which contains information on the target cell so that the wireless device may access the target cell.
(3) When the wireless device receives the RrcReconfiguration message from the source network node, it will start to access to the target cell, and it will then send handover complete to the source network node after radio link setup success in the new, i.e., target, cell.
If the parameters offset and time to trigger (as shown in FIG. 1) are not set properly, handover may happen too early or too late, which will probably result in a handover failure.
In a cell edge, if a target, also referred to as neighboring, cell is using the same frequency band as the source, also referred to as serving, cell, there may be strong intra-frequency interference. This interference may lead to low Signal-to-Noise Ratio (SNR) when a wireless device is on the cell edge during a handover process, and this may in turn lead to handover failure.
Handover failure is a problem which will degrade the user experience. In practice, various handover failures may occur, and these failures may be grouped into four categories:
(1) Too early handover;
(2) Too late handover;
(3) Handover that is not triggered properly; and
(4) Ping-ponging handover.
FIG. 2 illustrates a first example in a practical scenario of a method of the prior art wherein the handover is performed too late. In the Figure, each one of the source network node and the target network node services three cells (A, B and C). A wireless device moves in the path and direction marked by the arrow.
If the mobility of the wireless device is greater than the handover parameter settings allow for, handover may be triggered when the signal strength of the source cell serviced by the source network node is too low. The signal quality may then not be good enough for the wireless device to detect the Radio Resource Control (RRC) Reconfiguration message. This may lead to a Radio Link Failure (RLF) in the source network node and re-establishment of the communication in a different cell than the source, usually in the target cell serviced by the target network node.
The general solution in current systems is to use Self-Optimizing Network (SON) to optimize the parameters related to handover and try to avoid handover failure resulting from too late handover.
This solution can only reduce the possibility of too late handover, and it needs to collect data for a long time, and analyze it repeatedly.
FIG. 3 depicts a second example in a practical scenario of a method of the prior art wherein the handover is performed too early. As in FIG. 2, each one of the source network node and the target network node may service three cells (A, B and C).
Too-early handover may be triggered when a wireless device enters an island of coverage of another cell contained inside the coverage area of the source cell. This is a typical scenario for areas where fragmented cell coverage is inherent to the radio propagation environment, such as dense urban areas.
In this example, the wireless device has contiguous coverage from a cell B, serviced by the source network node, but due to a region of strong coverage from a cell A, serviced by target network node, a handover takes place to the cell A of the target network node. As the wireless device exits this region of coverage, an RLF occurs and the wireless device reconnects to the cell B of the source network node.
According to prior methods using SON, cell A of the target network node analyses the RLF and checks the length of time it carried the call. If cell A of the target network node carried the call for less than a certain period of time, it then concludes that the wireless device should not have been handed over to cell A of the target network node in the first place. The target network node then sends a Handover Report to the source network node and the neighbor relationship from cell B of the source network node to cell A of the target network node is adjusted such that the wireless device does not handover to cell A of the target network node in the first place. This prior art method requires time consuming data collection and analysis.
Currently, there are many ways to try to overcome the above handover problems. These solutions mainly focus on optimization of handover parameters based on feedback information in an RLF and RRC re-establishment from the wireless device and uplink measurement from the source cell. According to the information from the wireless device and the source network node, at least one of the source network node and the target network node may be able to identify the different scenarios of handover failure, such as too early handover, too late handover and so on. The optimization function will then adjust the corresponding parameters based on these inputs.
US8012/0088507A1 provides a way to identify different handover problems. The approach disclosed in this application provides full information across the handover process for the handover problem identification and parameter adaptation, uplink and downlink measurements before handover trigger and after the handover is complete. Measurements for handover failure and handover success are all included. Based on the input information, the performance of handover identification and parameter adaption is improved.
US8010/0173626A1 provides a method for configuring adaptive handover parameters, time to delay and signal strength offset included. This mitigates the handover failure problems through repeated adjustment of these parameters.
All the above approaches focus on the handover failure identification and parameter optimization. However, these processes require a lot of information input. Moreover, in addition to the fact that the self-optimization process takes a long time, it can only reduce the possibility of handover failures because of too late handover and too early handover, but it cannot avoid it.